This information was revealed in The Lucy-Desi Center Gift Shop's Spring 2007 Merchandise Guide. The guide states (image above):

It's double the fun with this new "I Love Lucy" two-doll box set-featuring Lucy and Ethel in one of their most outlandish misadventures. The episode theme of this set is still top secret, but we're taking pre-orders now to guarantee you'll be one of the first to receive this limited edition when it is released.

Your credit card will not be charged until your order is shipped to you. Orders for this special doll set will be shipped in the order they are received. Quantity will be limited, so don't delay! Pre-order yours today.

No release date for the dolls was given. To preorder the "Lucy & Ethel Two-Doll Boxed Set" (#2340) for $89.95 (plus shipping and handling) call toll-free 1-877-582-9326. As of this writing, the dolls are not available for preorder from the gift shop online yet.

The Long Live Lucy Blog was the first Lucille Ball Blog to feature a photo of Mattel's 2007 "I Love Lucy" Barbie Collector Doll based on "The Audition." A photo of this new set will be posted as soon as possible.